Particulates (e.g., soot) may form in internal combustion engines as a byproduct of some combustion processes. For example, particulates may form in the exhaust gas at high engine speeds or high engine loads. The formation of particulates may also be related to the direct injection of fuel into engine cylinders. Particulate filters in the exhaust line may be used in order to retain the particulates and reduce soot emissions. Over time, the particulates accumulate within the filter, reducing the exhaust flow rate through the exhaust system and creating an engine back pressure which may reduce engine efficiency and fuel economy. To reduce the backpressure, the filter may be intermittently regenerated to burn off accumulated soot. However, even with intermittent regeneration, particulate filters may degrade and leak particulates to the atmosphere through the tailpipe.
One way to determine whether a particulate filter is leaking is through the use of pressure sensors, as shown by Yamakawa et al. in EP 2690263. Therein, a pressure value at an upstream side of a particulate filter and a pressure value at a downstream side of the particulate filter are Fourier-transformed and compared so as to detect an amount of particulates deposited on the particulate filter. The health of the filter is then determined based on the comparison. Another way to determine whether the particulate filter is leaking is shown by Yadav et al. in US 2012/0125081. Therein, the accumulation of particulate matter is determined based on a particulate sensor coupled with a temperature sensor and a flow velocity sensor placed in the exhaust line downstream of the filter. In response to a particulate sensor input value and a particulate sensor temperature, a controller determines a particulate filter diagnostic value. The particulate filter diagnostic value being above a specific value within a predetermined time indicates the particulate filter is leaking soot out to the exhaust.
However, the inventors herein have identified potential issues with such approaches. As one example, in the approach of Yamakawa, there may be an additional cost associated with adding pressure sensors to the exhaust system. In addition, pressure sensors may not be durable in the harsh conditions of the exhaust system, requiring frequent replacement. Further, the differential pressure between the upstream and downstream pressure sensors may need to be substantially different to indicate particulate filter degradation. As another example, in the approach of Yadav, the particulate sensors may need frequent regeneration. As such, the monitoring of the change in amount of particulate matter may need to be done after filter regeneration so as not to corrupt the results of the diagnostic routine. Consequently, there may not be sufficient opportunities for ongoing particulate filter monitoring. Still other issues include the need for extra sensors, such as temperature sensors, which add component cost and control complexity.
The inventors herein have recognized the above mentioned issues and developed a method for determining gasoline particulate filter (GPF) leakage, or degradation, in an exhaust system. The method comprises, during selected conditions, correlating an output of an upstream exhaust oxygen sensor and a downstream exhaust oxygen sensor with a pressure drop across an exhaust particulate filter. The pressure drop may then be correlated with leakage from the particulate filter. The selected conditions may include conditions where the exhaust oxygen concentration across the filter remains substantially constant. In this way, existing exhaust oxygen sensors can be used as pressure sensors during selected conditions, allowing for changes in the partial pressure of exhaust gas oxygen across the filter to be correlated with GPF health
As an example, an exhaust system may include a first exhaust gas sensor (e.g., first oxygen sensor) located upstream of an exhaust gasoline particulate filter (GPF) and a second exhaust gas sensor (e.g., second oxygen sensor) located downstream of the GPF. During engine operating conditions, such as regeneration and oxygen uptake in the GPF, the output of the first pre-GPF oxygen sensor and the second post-GPF oxygen sensor may be used to infer filter soot levels. In particular, a change in oxygen concentration across the filter may be correlated with soot mass oxidized within a particulate filter. As such, since the exhaust gas sensors measure a partial pressure of exhaust oxygen, during selected engine operating conditions where the oxygen concentration across the particulate filter remains substantially constant, such as during engine cold start and after filter regeneration, the output of the sensors may differ. Specifically, the upstream oxygen sensor may have a higher output than the downstream sensor. During those conditions, a correction factor based at least on exhaust gas flow rates, may be calculated and applied to correct the sensor outputs. If after correction, a difference between the sensor outputs monitored over a defined time interval is lower than a threshold (e.g., if the corrected output of the pre-GPF exhaust sensor is lower than the output of the post-GPF exhaust sensor), the engine controller may infer that the change in partial pressure across the filter is due to filter degradation. For example, it may be inferred that the GPF is leaking and a diagnostic code may be set.
In this way, existing exhaust gas sensors may be advantageously used during selected conditions to infer particulate filter leakage without the need for other dedicated sensors, such as dedicated pressure or temperature sensors. By monitoring the output of exhaust gas oxygen sensors that are sensitive to the partial pressure of oxygen at operating conditions where the exhaust oxygen concentration across a GPF is not changing, the oxygen sensors may be advantageously used as pressure sensors. A pressure change across the filter, estimated based on the output of the exhaust gas sensors, may then be correlated with filter health. For example, the output of the sensors may be compared after filter regeneration, during engine steady state conditions, and/or after an engine cold start to identify filter degradation based on differences in oxygen partial pressure across the filter. By using components already available in the engine system, component reduction benefits are achieved without reducing the reliability of the results of the diagnostic routine. By monitoring particulate filter health, vehicle emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.